1. Field of the Invention
This invention relates to blowdown apparatus in general and more specifically to a blowdown apparatus for use in a once through steam generator.
2. Description of the Prior Art
The practice of blowing down boiler water in a recirculating steam generator to effectuate the removal of solid contaminants entrained therein is well known.
Due to the nature of recirculating steam generators, solids introduced into the generator by entering feedwater tend to concentrate within the recirculating boiler water rather than exiting with the generated steam. This undesirable state of affairs occurs, in part, due to the presence of a generally fixed steam-water inferface located within the generator. The solubility ratio between the steam and water phases at the interface results in essentially all of the soluble feedwater solids being retained in the water phase. Although maximum solids concentration will occur at the interface, a significant quantity of entrained solids will be dispersed throughout the boiler water as well. Ultimately, their continuing presence within the water will simultaneously reduce the heat transfer efficiency of the generator, promote debilitating corrosion within the steam generator and increase the carryover of solid contaminants entrained within the exit steam.
The problems previously discussed are further compounded by the fact that fresh feedwater entering the generator is constantly introducing small amounts of contaminants to the boiler water which is in addition to the contaminants already present. Left unchecked, contaminant buildup will occcur at a rapid rate.
To alleviate this problem, a portion of the boiler water is removed or "blown down" either at specific time intervals or continuously. Since the solids' concentration in the boiler is significantly greater than that of the feedwater entering the generator, the blowdown flow need only be a fraction of the feedwater flow to maintain contaminant levels within acceptable levels.
In contrast to a recirculating steam generator, a once through steam generator (OTSG) does not experience contaminant buildup at any one fixed location. This occurs because the fixed steam-water interface, always present in a recirculating steam generator, is absent when the OTSG operates at high load levels. Instead, the entrained contaminants are transferred to the exiting steam at essentially the same rates as they are introduced into the generator. As a consequence, blowdown is unnecessary at high loads. Unfortunately, when a OTSG is operated at low power levels, a steam-water interface will develop within the generator. However, in contrast to a recirculating steam generator, the position of the steam-water interface in a OTSG will vary as a function of the load impressed upon the generator. As a consequence, the previously discussed problems engendered by the steam-water interface occurring within the recirculating steam generator will manifest themselves in the OTSG as well, even though the water level may vary. Therefore, it is desirable to provide an OTSG with a universal blowdown apparatus which will eliminate contaminant buildup regardless of the level of the steam-water interface.